The invention relates to image devices in general. More particularly, the invention relates to a method and apparatus for minimizing burn lines on a cathode ray tube (CRT) display used in an image display device such as a television.
The presentation of moving images on a display has been a popular medium of communication for many years. As a result, many innovations have evolved to make movies and films capable of reproducing the visual and audible depth, robustness and acuity of the human senses. For example, digital image processing and communication techniques are capable of producing images and movies of exceptional high quality.
As with many industries, the evolution of technologies presents problems, particularly with respect to transitioning from an older technology to a newer technology. For example, in the wireless communications industry, much of the existing infrastructure and cellular telephones were designed to carry analog signals. Due to quality and capacity issues, digital systems and digital cellular telephones were designed. Since development of an entirely new digital system would take a number of years, however, an interim cellular telephone was developed that was capable of communicating using both analog and digital signals (sometimes referred to as xe2x80x9cdual-modexe2x80x9d telephones).
The television industry is currently facing a similar problem. In particular, the television industry is presently transitioning from an existing video standard referred to as the National Television Systems Committee (NTSC) standard or the Electronic Industries Association (EIA) standard RS-170A developed in the late 1940s to the High Definition Television (HDTV) standard finalized in the early 1990s. A video signal xe2x80x9cstandardxe2x80x9d specifically prescribes a video signal""s synchronization timing, electrical voltage levels, and quality measures.
As a result, many television manufacturers are now developing televisions that are cable of displaying the present NTSC standard signals as well as the new HDTV standard signals. One key difference between these two standards, however, is the aspect ratio. The term xe2x80x9caspect ratioxe2x80x9d refers to the dimensions of an image or picture which is calculated by dividing the image""s horizontal width by its vertical height. In the present NTSC standard, the aspect ratio is a xe2x80x9c4:3xe2x80x9d aspect ratio, where 4 is the horizontal dimension and 3 is the vertical dimension. In other words, the horizontal dimension of the image is 1.33 times wider than the vertical dimension. By way of contrast, the HDTV standard calls for a xe2x80x9c16:9xe2x80x9d aspect ratio. When a 16:9 aspect picture is displayed on a 4:3 aspect ratio television, the horizontal width of the 16:9 aspect picture matches the horizontal width of the 4:3 aspect ratio television, but not the vertical height. Consequently, black bands appear at the top and bottom of the television screen. This is sometimes referred to as a xe2x80x9cletterboxxe2x80x9d effect, and is shown in FIG. 1.
FIG. 1 is a picture of a 16:9 aspect ratio picture displayed on a 4:3 aspect ratio television display. Displaying a 16:9 aspect picture on a 4:3 aspect ratio television creates a problem referred to as CRT burning. This burning is caused by the phosphors of the CRT being active in the 16:9 portion (shown in white in FIG. 1 and denoted with number 106), but not active in the border area (shown in black in FIG. 1 and denoted with numbers 102). As the phosphors are activated they begin to age and produce less brightness. Thus, the phosphors in the active 16:9 portion begin to age at a rate that is faster than the inactive border area, causing a difference in brightness output that is most noticeable at the transition points (lines between white 106 and black 102 denoted by numbers 104). If the 4:3 aspect ratio television is used to display only 16:9 aspect ratio signals, the burning at the transition points cannot be visually detected. If the television is used to display a 4:3 aspect ratio picture, after sufficient amount of time displaying a 16:9 aspect ratio picture, however, it is possible to visually detect a burn line (lines 104) along the top and bottom edges of the 16:9 aspect ratio picture. This effect is shown in FIG. 2.
FIG. 2 is a picture of CRT burn lines caused by displaying a 16:9 aspect ratio picture on a 4:3 aspect ratio television. As demonstrated in FIG. 2, when a 4:3 aspect ratio picture is once again displayed on the 4:3 aspect ratio television, a pair of noticeable burn lines 202 appear at the top and bottom edges of the previously displayed 16:9 aspect ratio picture. These noticeable horizontal burn lines 202 adversely effect picture quality and distract a viewer.
Another problem associated with displaying a 16:9 aspect ratio picture on a 4:3 aspect ratio display occurs when a 4:3 aspect ratio picture is once again displayed on the 4:3 aspect ratio television. More particularly, the border area created by the 16:9 aspect ratio picture is not active during the display of the 16:9 aspect ratio picture. Because the border area is inactive, the phosphors are not aging at the same rate as the area used for the 16:9 aspect ratio picture. Consequently, when a 4:3 aspect ratio picture is once again displayed on the 4:3 aspect ratio screen, the border area looks brighter than the area used for the 16:9 aspect ratio picture.
At least one previous attempt has been made to solve the CRT burning problem. This attempt, however, is less than satisfactory for a number of reasons. For example, the counter measure attempts to avoid the CRT burning problem by using deflection waveforms to make the 16:9 aspect ratio picture cover the full 4:3 aspect ratio screen. In other words, the 16:9 aspect ratio picture is vertically stretched to cover the black bands at the top and bottom of the 4:3 aspect ratio television display. If the deflection is linear, however, objects in the 16:9 aspect ratio picture appear disproportionately taller than normal. If the deflection is non-linear, shape distortion occurs in the 16:9 aspect ratio picture. Thus, in both instances, the displayed image is noticeably distorted or different from the original image.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus capable of displaying a 16:9 aspect ratio signal on a 4:3 aspect ratio television that minimizes CRT burning without distorting the 16:9 aspect ratio picture.
One embodiment of the invention comprises a method and apparatus for displaying a video signal. A video signal is received representing an image having a first aspect ratio, with the image having a top and bottom edge. An edge modification signal is generated. The image is displayed on a display having a second aspect ratio with the top and bottom edge modified in accordance with the edge modification signal. Accordingly, burn lines are minimized through use of the edge modification signal to reduce brightness levels at the top and bottom edge of the image.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.